I haven’t heard people using those that much. I guess the concave spokeshave could work.
I use quite little of concavity so i need just few passes with normal spokeshave. Then the final concavity is actually made by the toothed scraper to get perfect match. So i want to stop rounding of the maple core a little bit earlier.
Tuomo uses this kind of rasp for making the horn convex. I might need to buy it too at some point. Glue to water ratio - #10 by TuomoReiniaho
We use same amount of concavity on our bows with Tuomo.